1. Field of the Invention
The present invention relates to a personal branch exchange telephone system and, more particularly, to interface circuitry for use in such a system to couple the system to outside telephone lines as well as to individual telephone sets on extension lines in the PBX system.
2. Description of the Prior Art
Personal branch exchanges (herein PBX's) are telephone system networks which are installed in business offices and similar environments to provide intra-office telephone communication as well as communication between the extension lines in the office and out-going telephone lines to remote locations. With such systems it is necessary to provide circuitry to interface between the PBX system extension lines and the telephone lines such that communication signals are faithfully conveyed into and out of the PBX system. Also, the interface circuitry should be such as to protect the PBX system from any potentially harmful electrical signals carried by the telephone's lines; for example, it is necessary to protect the PBX system from high voltage surges caused by lightning strikes and the like incurred by the telephone lines. Conversely, such interface circuitry should be designed so that high voltage signals are not conveyed from the PBX system to the telephone lines, thereby to avoid damage to the switching equipment associated with the telephone lines, the telephone lines themselves, or personnel servicing the telephone lines.